Dielectrophoresis (“DEP”) refers to the force experienced by particles suspended in a fluid medium in applied electric field gradients. The dielectrophoretic force is, generally, speaking, the interaction of a non-uniform electric field with the dipole moment it induces in an object. The typical case is the induced dipole in a lossy dielectric spherical particle. The concept of dielectrophoresis has been explained in Pethig, Review Article—Dielectrophoresis: Status of the theory, technology, and applications, Biomicrofluidics 4, 022811 (2010). Dielectrophoresis can be used to manipulate, transport, separate, and sort different types of particles. Since biological cells have dielectric properties, dielectrophoresis has many potential medical applications. See, e.g., Pethig et al., Dielectrophoresis: A Review of Applications for Stem Cell Research, Journal of Biomedicine and Biotechnology, Vol. 2010, 182581 (doi: 10.1155/2010/182581).
More particularly, DEP may be used for characterizing cells by measuring the changes in their electrical properties. When an electric field gradient is generated, differences in the dielectric polarization between the particles and the fluid medium cause the particles to experience a dielectrophoretic force. This effect can be quantified in terms of the electromagnetic momentum balance via the Maxwell stress tensor, or in terms of the magnitude and distribution of the charges induced on and within the particle by the applied field. Particles, such as blood cells, experiencing strong DEP motion will typically experience a DEP force of about 1×10−11 N, which is about 40 times greater than the gravitational settling force and about 2×105 times larger than the Brownian diffusion force.
A particle's structural and physico-chemical properties can contribute towards its DEP response. Additionally, the DEP response can also depend on, inter alia, the frequency of the applied electric field. Due to these various dependencies, variations in applied field frequencies and external environment can be used to simultaneously probe different particle substructures and processes. For example, some fundamental electrical properties of cells, such as membrane capacitance, membrane resistance, and cytoplasmic conductance can affect their DEP response. These properties also reflect a cell's ability to maintain ion balances and are a measure of metabolic work and biological organization. Thus, DEP can provide a relatively non-invasive procedure for determining the electrical properties of cell populations, down to the single cell level, and reveal important information about the cells.
The frequency dependence and the direction of the DEP force are governed by the real part of the Clausius-Mossotti factor, which indicates the relative polarizability of a particle with respect to its suspending medium. If a particle, or population of particles, is more polarizable than the suspending medium, then the particle(s) will be attracted to high-intensity electric field regions. This is termed as positive dielectrophoresis (pDEP). Conversely, if a particle, or population of particles, is less polarizable than the suspending medium, the particles will be repelled from the high-intensity field regions, and negative dielectrophoresis (nDEP) occurs. Therefore, the real part of the Clausius-Mossotti factor characterizes the frequency dependence of the DEP force.
Various methods have been developed to measure the DEP cross-over frequency as a function of the conductivity of the suspending medium to provide information for assessing the dielectric properties of suspended particle(s). The DEP cross-over frequency, (fxo or fcross), the transition frequency point where the DEP force switches from pDEP to nDEP, or vice versa. Determination of frequency cross-over, cell diameter, along with the conductivity of a suspending solution, provides a measure of cell membrane capacitance. In addition to measurements of crossover frequency, the DEP-induced particle velocity can be measured to assist in characterizing particles. The DEP-induced particle velocity is directly proportional to the DEP force.
Various techniques are available to quantify the dielectrophoretic response of a particle, such as a cell. Systems have been designed to expand and improve applications, efficiencies, reproducibility, and reliability of various types of particle separations, and examinations. See, e.g., Gupta et al., ApoStream™, a new dielectric device for antibody independent isolation and recovery of viable cancer cells from blood, Biomicrofluidics 6, 024133 (2012); and Lee et al., U.S. Pat. No. 7,063,777, entitled “Dielectrophoretic Particle Profiling System And Method.”
However, large variations in the percentage or degree of separation of particles for a given sample can arise when employing DEP devices. This is because, while there are general guidelines regarding the frequency of the signal that should be employed to separate certain types of particles from others, e.g., certain types of cancerous cells from non-cancerous cells in a sample, in actual practice, particularly in biological applications, there can be significant variation in the voltage frequency that should be employed to separate them from non-target particles in a sample of interest. Thus, in actuality, it can be difficult to obtain a high and consistently reliable degree of separation between target particles and non-target particles. This is particularly important in medical applications, where a consistently high degree of separation of target particles from non-target particles can be critical to accurate medical assessments and treatments of potential patients.
Experimental procedures have been devised in attempts to determine cross-over frequency of particles of interest. These include applying small alternating current (AC) frequency increments or decrements to an applied voltage so that cells in a suspension undergo a cross-over frequency event. Theoretically, the cross-over frequency can be described as the frequency at which a cell undergoes no movement. However, in actual practice, the frequency increments or decrements that are applied can be too large to visually observe the point of no movement of a particle of interest. However, since frequency changes result in either positive DEP (PDEP) or negative DEP (NDEP) and not zero movement, it is difficult to accurately identify the point at which a cross-over event happens.